check integrity of game files have you tried that?
if you didn't right click on besiege in steam on the side bar with all your games and go to properties local files and then verify integrity of game files...
a option to fill balloons with a denser gas then air so you could have negatively weighed balloons
pressurized tanks for lift like fill the tank with hydrogen and if hit by lets say a arrow can explode but provides more lift than the generic balloon.
infantry for your team the...